Radio communication system, method of operating a communication system, and a mobile station

ABSTRACT

In a mobile communication system comprising a base station ( 100 ) and a plurality of mobile stations ( 200 ) and operating closed loop transmitter power control, power control commands for transmission on an uplink are derived from measurements made on received downlink signals comprising nonpredetermined data values. Optionally the non-predeterrnined data values may comprise power control commands for uplink transmit power control.

The invention relates to a radio communication system, a method ofoperating a communication system, and a mobile station for use in acommunication system.

In a radio communication system comprising base stations of fixedlocation and mobile stations, transmissions from a base station to amobile station take place on a downlink channel and transmissions from amobile station to a base station take place on an uplink channel. It isknown to use downlink closed loop transmit power control in which amobile station measures the quality of a received power controlleddownlink pilot signal and transmits transmit power control (TPC).commands to a base station so that an adequate, but not excessive,received signal level is maintained at the mobile station despitefluctuations in downlink channel conditions. It is also known to useuplink closed loop transmit power control in which a base station.measures the quality of a received uplink pilot signal and transmitstransmit power control (TPC) commands to a mobile station so that anadequate, but not excessive, received signal level is maintained at thebase station despite fluctuations in uplink channel conditions.

When a plurality of mobile stations share a channel, separate TPCcommands are provided for each active mobile station because each mobilestation will experience unique channel fluctuations. Similarly, aseparate downlink pilot signal is provided for each active mobilestation; each mobile station demodulates its respective pilot signal toestimate the channel characteristics prevailing for that mobile stationand optionally to generate a phase reference. The estimated channelcharacteristics and the phase reference are then used to assistdemodulating information carrying signals. The pilot signals comprisepredetermined data values to enable a mobile station readily todetermine distortion introduced by the channel.

The pilot signals and the signals conveying the TPC commands are subjectto transmit power control.

The transmission of the pilot signals and the TPC commands uses systemresources. For example, in a Code Division Multiple Access (CDMA) systemchannel codes are required for the pilot signals and TPC commands, andin a Time Division Multiple Access (TDMA) system time slots are requiredfor the pilot signals and TPC commands.

An object of the invention is to reduce the requirement for systemresources.

According to a first aspect of the invention there is provided a mobilestation for use in a communication system having a base station, themobile station comprising:

-   receiver means for receiving from the base station a first downlink    signal, measurement means for measuring a parameter of the received    first downlink signal;-   power control means for generating first power control commands in    response to the measured parameter; and-   transmitter means for transmitting the first power control commands    to the base station;-   wherein the measurement means is adapted to measure the parameter of    the first downlink signal while first downlink signal is modulated    with non-predetermined data values and is subjected to transmit    power control in accordance with the first power control commands.

According to a second aspect of the invention there is provided a radiocommunication system comprising a base station and at least one mobilestation in accordance with the first aspect of the invention.

According to a third aspect of the invention there is provided a methodof operating a communication system comprising a base station and atleast one mobile station, comprising

-   at the base station, receiving first power control commands    transmitted by the mobile station and transmitting a first downlink    signal modulated with non-predetermined data values and subjected to    transmit power control in accordance with the first power control    commands, and-   at the mobile station, receiving the first downlink signal,    measuring a parameter of the first downlink signal modulated with    the non-predetermined data values, generating the first power    control commands in response to the measured parameter, and    transmitting the first power control commands.

The invention is based on the realisation that downlink closed looppower control may be operated by measuring the quality of receiveddownlink nor-predetermined data symbols instead of predetermined pilotsymbols, and that in some circumstances, separate downlink pilot signalsfor each active mobile station are not necessary for channel estimation.In some circumstances, downlink channel estimation is not required atall, and in other circumstances a common downlink pilot signaltransmitted at a constant power level may be used instead of separatepilot signals. Consequently, operation is possible using fewer downlinksystem resources.

Optionally, the non-predetermined data values used for measuring thequality of a received signal for downlink closed loop power control mayconvey downlink TPC commands used for uplink power control.

The invention will now be described, by way of example only, withreference to the accompanying drawings wherein:

FIG. 1 is a block schematic diagram of a communication system; and

FIG. 2 is flow chart of a method of operating a communication system.

One application for the invention is in the Universal MobileTelecommunication System (UMTS). In the UMTS Frequency Division Duplex(FDD) mode, in Release 5 of the UMTS Specifications which may be viewedat http://www.3gpp.org, it is possible to operate High Speed DownlinkPacket Access (HSDPA) in such a way that a downlink dedicated channel isnot needed for data (user or signalling), as data can be sent via theHigh Speed Downlink Shared Channel (HS-DSCH). A dedicated channel uses asingle channel code for exactly one user, while a shared channel allowsa plurality of users to share a single channel code, eithersimultaneously, or in rapid time multiplex. Even though data can be sentvia a shared channel, a downlink Dedicated Channel (DCH) is stillrequired for transmitting TPC commands for each active mobile station,in order to control the uplink transmit power.

The existence of a downlink DCH requires the allocation of a channelcode for the duration of the connection. One way of operating thedownlink DCH is to configure it as a fractional DCH which comprises onlypilot symbols and TPC commands, with multiple users multiplexed on tothe same channel code in such a way that each user uses the channel codefor only a fraction of each timeslot. OSignalling is used to assignmobile stations to use a particular channel code and fraction of atimeslot, in order to align the uplink and downlink power controltiming. Such a scheme frees up channel codes which can be used toincrease system capacity. However the present invention requires evenfewer resources.

The requirement is for the mobile station to be able to derive a powercontrol command to send in the uplink. This will then be used by thebase station to adjust the power of the part of the fractional DCHcorresponding to that mobile station.

The invention is based on the recognition that separate pilot symbolsfor each active mobile station are not necessary in at least two cases:

-   1) Where the transmitted phase of the DCH is referenced to that of a    common pilot signal, for example by using the same antenna(s) and    antenna weights for both the DCH and the common pilot signal to    which the phase of the DCH is referenced. In this case the    characteristics of the radio channel can be estimated from the    common pilot signal and this estimate can be used to demodulate the    TPC bits.

This first case is very likely to apply in HSDPA, as the HS-DSCH will beassigned a common pilot signal as a reference and the same common pilotsignal can be used for the fractional DCH. Since the total power used byfractional DCH's is not likely to be very large, the benefits ofseparate antenna beamforming for fractional DCH will not be large.

-   2) Where different antennas or antenna weights are used for the    common pilot signal and the DCH, but the correlation between them is    sufficiently good that the common pilot signal can be used to make a    reasonable channel estimate for the DCH, such that the data on the    DCH can be received reliably.

So, in accordance with the invention the downlink fractional DCH canconsist only of non-predetermined information bits multiplexed betweenusers. A special case of particular interest is where these informationbits carry TPC commands. The amplitude of individual TPC bits may beadjusted by the base station according to power control commandsreceived from the relevant mobile station. The mobile station determinesthe radio channel phase characteristics from an appropriate common pilotsignal, demodulates the TPC commands, and increases or decreases themobile station uplink DPCCH power as required. In addition, the mobilestation uses the amplitude of the received TPC bits to determine any TPCcommands sent in the uplink.

FIG. 1 is a block schematic diagram of a communication system comprisinga base station 100 and a mobile station 200. In practice there would bea plurality of mobile stations 200, but for clarity only a single mobilestation 200 is illustrated.

The mobile station 200 comprises a receiver 220 coupled to an antenna210 for receiving radio signals transmitted by the base station 100.Coupled to an output of the receiver 220 is a measurement means 250 formeasuring a parameter of a signal received from the base station 100.The measurement means 250 is adapted to measure the parameter of asignal which is modulated with non-predetermined data and is subjectedto transmit power control by the base station 100. An output of themeasurement means 250 is coupled to a first input of a power controlmeans 230. The power control means 230 comprises a generation means (TPC1) 232 for generating first TPC commands in response to the parametermeasured by the measurement means 250. A first output of the powercontrol means 230 is coupled to a first input 244 of a transmitter 240for transmitting the first TPC commands via the antenna 210 to the basestation 100.

The base station 100 comprises a transmitter 140 and a receiver 120coupled to an antenna 110. The receiver 120 receives radio signalstransmitted by the mobile station 200, in particular the first TPCcommands. An input of a power controller (PC) 130 is coupled to anoutput of the receiver 120 for decoding the first TPC commands receivedfrom the mobile station 200, and a first output of the power controller130 is coupled to a first input 142 of the transmitter 140 forcontrolling the transmit power of the transmitter 140 in accordance withthe first TPC commands.

Optionally, at the base station 100, the power controller 130 measuresthe quality of a signal received from the mobile station 200 and, inresponse to the measured quality, generates second TPC commands. Asecond output of the power controller 130 is coupled to a second input144 of the transmitter 140 for transmission of the second TPC commandsto the mobile station 200, to control the transmit power of the mobilestation 200.

Optionally, at the mobile station 200, the power control means 230 iscoupled to an output of the receiver 220 and comprises a decoding means(TPC 2) 234 for decoding the second TPC commands. A second output of thepower control means 230 is coupled to a second input 242 of thetransmitter 240 for controlling the transmit power of the transmitter240 in accordance with the second TPC commands. In this case, thenon-predetermined data on which the measurement means 250 makes itsmeasurement may be the second TPC commands.

Referring to FIG. 2, steps on the left hand side of the flow chartrelate to steps performed at the base station 100, and steps on theright hand side of the flow chart relate to steps performed at themobile station 200.

At step 310 the base station 100 receives first TPC commands from themobile station 200.

At step 320 the base station 100 transmits to the mobile station 200 asignal modulated with non-predetermined data values and subjected totransmit power control in accordance with the first TPC commands.

At step 330 the mobile station 200 receives the signal modulated withthe non-predetermined data values.

At step 340 the mobile station 200 measures a parameter of the signalmodulated with the non-predetermined data values.

At step 350 the mobile station 200 generates the first TPC commands inresponse to the measured parameter.

Optionally, at step 360, the non-predetermined data values may comprisesecond TPC commands and the mobile station 200 may decode the second TPCcommands, and at step 370 the mobile station may control its transmitpower in accordance with the second TPC commands.

At step 380 the mobile station 200 transmits the first TPC commandsgenerated at step 350.

Flow then reverts to step 310 and the process continues to loop. Thesignal parameter measured by the measurement means 250 of FIG. 1, and atstep 340 of FIG. 2, may be, for example, any of signal to noise ratio;signal to interference ratio; E_(b)/N₀ (where E_(b) is energy per bitand N_(o) is noise density).

The decision threshold for TPC commands sent in the uplink can bedetermined by setting a particular error rate requirement for the TPCcommands received in the downlink. This is in contrast to currentpractice which is to set a decision threshold in terms of the signal tonoise ratio required to meet a predetermined performance target, forexample a frame error rate of a data channel. One application for theinvention is a fractional control channel in UMTS FDD (frequencydivision duplex) mode. With a spreading factor of 256 there are 10symbols per slot. Therefore one slot can conveniently support either 2,5 or 10 users with 5, 2 or 1 symbol per TPC command respectively.

In another application of the invention in UMTS the spreading factorwould be 128, which would support the use of Space Time TransmitDiversity (STTD) applied to groups of two symbols. In this case, thereare 20 symbols per slot, so 10 users can be supported while stillmaintaining the 2 symbols per TPC command which are required for theSTTD encoding process.

Optionally, by avoiding the need to transmit a separate pilot signal foreach user, the energy that would have been used to transmit the separatepilot signals may be redeployed by increasing the number of symbols usedto transmit the TPC commands, thus improving the reliability of the TPCcommands.

In the present specification and claims the word “a” or “an” precedingan element does not exclude the presence of a plurality of suchelements. Further, the word “comprising” does not exclude the presenceof other elements or steps than those listed.

From reading the present disclosure, other modifications will beapparent to persons skilled in the art. Such modifications may involveother features which are already known in the art of mobilecommunications and which may be used instead of or in addition tofeatures already described herein.

1. A mobile station (200) for use in a communication system having abase station (100), the mobile station (200) comprising: receiver means(220) for receiving from the base station (100) a first downlink signal,measurement means (250) for measuring a parameter of the received firstdownlink signal; power control means (230) for generating first powercontrol commands in response to the measured parameter; and transmittermeans (240) for transmitting the first power control commands to thebase station (100); wherein the measurement means (250) is adapted tomeasure the parameter of the first downlink signal while first downlinksignal is modulated with non-predetermined data values and is subjectedto transmit power control in accordance with the first power controlcommands.
 2. A mobile station (200) as claimed in claim 1, wherein thereceiver means (220) is adapted to receive from the base station asecond, non-power controlled downlink signal and to derive a channelestimate from the second downlink signal, and to employ the channelestimate to decode the first downlink signal.
 3. A mobile station asclaimed in claim 1 or 2, wherein the power control means (230) isadapted to decode the non-predetermined data values comprising secondpower control commands and to adjust the transmit power of thetransmitter means in accordance with the decoded second power controlcommands.
 4. A radio communication system comprising a base station(100) and at least one mobile station (200) as claimed in any of claims1,2, or
 3. 5. A radio communication system as claimed in claim 4, thebase station (100) comprising a receiver means (120) for receiving thefirst power control commands and a transmitter means (140) fortransmitting the first downlink signal modulated with non-predetermineddata values and subjected to transmit power control in accordance withthe first transmit power control commands.
 6. A method of operating acommunication system comprising a base station (100) and at least onemobile station (200), comprising at the base station (100), receivingfirst power control commands transmitted by the mobile station (200) andtransmitting a first downlink signal modulated with non-predetermineddata values and subjected to transmit power control in accordance withthe first power control commands, and at the mobile station (200),receiving the first downlink signal, measuring a parameter of the firstdownlink signal modulated with the non-predetermined data values,generating the first power control commands in response to the measuredparameter, and transmitting the first power control commands.
 7. Amethod as claimed in claim 6, comprising at the base station (100),transmitting a second downlink signal at a constant power level, and atthe mobile station (200), receiving the second signal, deriving achannel estimate from the second downlink signal, and employing thechannel estimate to decode the first downlink signal.
 8. A method asclaimed in claim 6 or 7, comprising at the base station (100), arrangingfor the non-predetermined data values to comprise second power controlcommands and, at the mobile station (200), decoding the second powercontrol commands and adjusting the transmit power of the mobile station(200) in accordance with the second power control commands.